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    John created the topic: Even Heating

    I was wondering if it is possible to get a very even, predictable heating of a specimen via microwave radiation? We are trying to make a device to heat little straws with biological material up to 36C (97F).

    My only experience with microwaves is the consumer microwave food ovens, and in them you get hot spots and cold spots of course. Is this inevitable? Or is there a way to make a more precision instrument to heat very evenly? One step further, if it is possible in theory: is there anyone in the industry that is making anything like this today that is available commercially?
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    Desert Sage replied to the topic: Even Heating

    I was hoping someone else would jump in here but here I go.

    The first question: Is it possible? Yes. A more important question would be is it practical?

    The second question: Is there anyone doing this commercially? Not to my knowledge.

    Here are my thoughts. You talk about little straws. After making sure they were all lined up (so I could use linear polarization) I would look for the resonant frequency of the little straws (and their filling) and pick a frequency accordingly (use resonance to my advantage). At that point it is relatively easy to control the power level driving the heating. It would be a much more difficult problem if the little straws were strewn about and/or they all have different resonance frequencies. I would suspect if the little straws were precisely manufactured to precise dimensions and coated with a lossy material (on the outside) the problem would get significantly easier. Good thermal conductivity (of the precision straws) would ensure there would be no hot/cold spots.
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    madengr replied to the topic: Even Heating

    Ha ha , I was about to jump in. Look into RF drying, as they place the material between capacative plates; i.e. in a uniform, near field.

    www.stalam.it/en/technologies/radio-freq...and-micro-wawes.html

    Problem is all these seem to be large systems. Not that you couldn't make one as multi-kW, 13.56 MHz sources are ubiquitous since they are used in semiconductor and vacuum processing.

    See Labotron RF3000:

    www.sairem.com/the-ovens-52.html

    As far as measuring the temp, a thermistor or thermocouple might work, but then you have to contend with high RF fields. You can get those liquid crystal tape thermometers (I have one on my beer brewing jug) and they are pretty precise. I assume they have some kind of formulation for each temperature. Or maybe an IR thermometer or fiber optic:

    www.omega.com/temperature/pdf/FOB100.pdf
  • Admin
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    John replied to the topic: Even Heating

    Desert Sage, thanks so much for your very helpful reply! I didn't know if anyone would respond.

    Desert Sage wrote: The first question: Is it possible? Yes.

    Woohoo! That's good news.

    A more important question would be is it practical?

    Yes, that is the bottom line question in the end.

    The second question: Is there anyone doing this commercially? Not to my knowledge.

    OK, good to know.

    After making sure they were all lined up (so I could use linear polarization) I would look for the resonant frequency of the little straws (and their filling) and pick a frequency accordingly (use resonance to my advantage). At that point it is relatively easy to control the power level driving the heating. It would be a much more difficult problem if the little straws were strewn about and/or they all have different resonance frequencies.

    No problem lining them up! The straws will be heated one at a time, and their location and orientation can be controlled to be the same every time. In fact, I was thinking that the cylindrical chamber that the straw slides into could itself be what I think you call the "reaction chamber" -- that is, the reaction chamber would be very small, just big enough for one straw to fit into. Straws are 5 inches long by 0.1 inches diameter, by the way.

    I would suspect if the little straws were precisely manufactured to precise dimensions and coated with a lossy material (on the outside) the problem would get significantly easier. Good thermal conductivity (of the precision straws) would ensure there would be no hot/cold spots.


    Now, I didn't make it clear, but I'm sure you guessed: it is actually the filling we care about heating, not the straw itself. The filling is biological; essentially an aqueous solution. So thermally, it will behave kind of like liquid egg whites. The straw is made of a PVC material. I am not sure what you mean by "a lossy material" -- lossy in what way, and is PVC lossy? The straws are definitely precision manufactured to set dimensions -- approx. +/- .002".
  • Admin
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    John replied to the topic: Even Heating

    madengr, all your links were very helpful. It looks like these Stalam fellows in Italy are the world leader in these RF drying machines, but, as you say, they are very big machines for industrial assembly lines and, giving you some idea of how big (and expensive) they are, they can only produce 100 per year.

    Your lead about making a homemade one from a 13.56 MHz source is interesting. Is this the kind of source you're talking about:

    www.newark.com/avx/k50-hc1cse13-5600mr/o...-5mm-cmos/dp/33K9509

    www.digikey.com/product-search/en/crysta...852334?k=13.56%20MHz
  • Admin
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    madengr replied to the topic: Even Heating

    No, those are just crystal oscillators. I was just suggesting 13.56 MHz as that frequency is used for high power industrial applications (it's actually an ISM frequency like microwave ovens). I figured you were wanting a microwave oven sized chamber, hence the requirement for a power amplifier such as:

    www.mksinst.com/product/category.aspx?CategoryID=17

    If you are just trying to heat a small tube to 97F, then you may only few 10's of watts.

    I would use Teflon instead of PVC for the tube; since PVC is somewhat lossy from my experience using it for radomes in the GHz range, but I have used it for coil-forms at HF with not too much loss. Unless you want to indirectly heat the fluid via the tube, but then why not just use immersion heating in a water bath, like those sous-vide cookers?

    It would be an interesting experiment to try, essentially a parallel plate capacitor, using Styrofoam as a sample holder. You'd need a transmatch between the amp and plates.
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